Technical Abstract:
Since the mid 1980's, the production of internally contaminated eggs by chickens infected with Salmonella enteritidis has been an important source of human illness on several continents. In response to this problem, substantial public and private funds have been spent on detecting and controlling S. enteritidis infections in commercial laying flocks. Although Salmonella serotypes other than S. enteritidis are also commonly found in the housing environment of egg-laying flocks, these other serotypes have rarely been found inside eggs or implicated in transmitting egg-borne disease. However, the Centers for Disease Control and Prevention have recently reported a significant association between eggs or egg-containing foods and S. heidelberg infections in humans. Using an experimental infection model that has previously been applied to document the deposition of S. enteritidis inside eggs, the present study determined if several S. heidelberg isolates could colonize reproductive tissues and thereby gain access to the interior contents of eggs laid by infected hens.
In each of two similar trials, three groups of 24 specific-pathogen-free laying hens were orally inoculated with doses of approximately 109 CFU of either an S. enteritidis strain (which caused egg contamination in several prior studies) or one of four S. heidelberg isolates that were originally obtained from egg-associated human disease outbreaks. Fecal samples were collected from all hens at weekly intervals. Internal organ samples were removed from euthanized hens at one and three weeks post-inoculation. The contents of all eggs laid during the first three weeks after inoculation were also sampled. All samples were cultured to detect S. enteritidis or S. heidelberg.
All S. enteritidis and S. heidelberg strains colonized the intestinal tracts of most inoculated hens and were shed in the feces at similar frequencies. Likewise, all five Salmonella strains invaded to reach the livers, spleens, ovaries, and oviducts of inoculated hens, with no significant differences observed between strains for any of these tissues. All five Salmonella strains were isolated from the liquid contents of eggs laid by infected hens, although the S. heidelberg strains were found at lower frequencies (ranging from 1.1% to 4.5%) than was the S. enteritidis strain (7.0% for the two trials combined).
This study demonstrates that some strains of S. heidelberg possess the ability to colonize the reproductive tract of chickens and can thereby be deposited inside eggs laid by these birds. However, as all four S. heidelberg strains used in these experiments were already associated with egg-transmitted human disease, the overall frequency at which these abilities are distributed among other strains of this serotype is not certain.